Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/09—Processes or apparatus for excitation, e.g. pumping
  • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
  • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
  • H01S3/094003—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light the pumped medium being a fibre
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
  • H01S3/06—Construction or shape of active medium

Definitions

• This invention relates to longitudinally pumped laser amplifiers of the type comprising either three or four level laser systems and particularly, but not exclusively, to optical fibre laser amplifiers.
• a laser amplifier may be formed by optically pumping an optical fibre doped with appropriate ionic species at a particular wavelength.
• Such laser amplifiers exhibit high pump power densities and good thermal dissipation and thus enable the operation of lasing with low pump power thresholds.
• continuous wave lasing in doped fibres has recently been achieved on 3-level laser transitions which had previously not been operated in bulk glasses at room temperature.
• erbium doped silica based optical fibres are of particular interest as laser sources and amplifiers.
• An ionic transition exists at about 1.5jum which coincides with the low-loss window of fused silica fibres.
• an erbium doped fibre has an absorption band at 800nm, the possibility exists for a variety of active fibre devices pumped by compact, efficient gallium arsenide laser diodes.
• the energy levels of some laser systems such as the erbium doped fibre described above are such that there exists at certain wavelengths suitable for optical pumping a parasitic phenomenon known as excited state absorption (ESA).
• ESA excited state absorption
• This is a process in which ions in the upper laser manifold of the system are excited by pump photons into higher energy states. These excited ions then relax non-radiatively back into the metastable manifold. This has the effect of reducing the proportion of pump photons which are usefully involved in producing a population inversion.
• the fraction of pump photons wasted is proportional to the relative populations of the upper and ground state manifolds. The greater the pump power, the greater the upper level population becomes, and hence the greater the effect of ESA. This results in the pumping process becoming more inefficient as the pump power is increased.
• the present invention provides a laser amplifier comprising: a single mode optical fibre, an optical pump source for providing optical radiation at a pump wavelength, and a plurality of fibre amplifier sections containing a lasing medium which exhibits excited state absorption at the pump wavelength in which the pump source is longitudinally coupled to each section so that each section is pumped by a fraction of the optical pump signal.
• An optical fibre of the type suitable for use in the present invention typically has a diameter of approximately 10 «m.
• the gain reduction associated with a given pump power may be reduced by dividing the pump power into two or more fractions and longitudinally pumping two or more fibre sections each with a fraction of the pump power thus increasing the pump efficiency. In this way, the pump energy distribution along the fibre is kept low, and hence the relative effects of ESA are reduced as will be explained in more detail below.
• the fibre amplifier sections may be adjacent portions of a single fibre, or they may each be a separate length of a fibre, optically coupled to each other.
• the sections are at least partially coincidental, the direction in which a first fraction of the pump power is pumped in one section being opposite to that in which a second fraction is pumped in the other section. This leads to additional gain being achieved because remnant pump power from one fibre amplifier section may be combined with the pump power in the second fibre amplifier section.
• the pump source is longitudinally coupled to the fibre by means of one or more dichroic fibre couplers.
• Such couplers are capable of coupling a pump signal whilst coupling the signal radiation into the same fibre.
• a laser amplifier comprising a three level laser system
• optimum length of fibre which, for a given pump power, will enable the maximum pump efficiency to be achieved. This optimum length depends upon the launched pump power and the pump absorption. For a low pump power, the optimum fibre length will be shorter than that for a higher pump power.
• each fibre amplifier section is substantially the optimum length for the pump power fraction at which it is being pumped.
• FIG. 1 is a schematic diagram of the energy level
• Figure 3 is a schematic diagram of one embodiment of the invention? and Figure 4 is a schematic diagram of a second embodiment of the invention.
• Er ions in a 3-level laser system have an energy level structure comprising a ground state 1, an intermediate etastable manifold 2 and a high state 3.
• the system comprises a pump band 4 at a wavelength of about 800 nm and an emission/absorption band 5 at a wavelength of about 1540 nm.
• erbium doped silica based fibre at 800nm
• erbium ions will be excited to the high state 3 and from there, they will decay to the metastable manifold 2.
• the energy level structure contains a further absorption band, the excited state absorption (ESA) band 6.
• This absorption band 6 exists between the metastable manifold 2 and a higher level 7.
• the ESA band occurs at a wavelength of about 800 nm which is substantially the same as the pump absorption band 4.
• ions are excited into the metastable manifold 2.
• These ions can then be excited to the higher level 7, from where they non-radiatively decay back to the metastable manifold 2.
• This has the effect of reducing the proportion of pump photons which are usefully involved in producing a population inversion.
• the fraction of pump photons wasted is proportional to the relative populations of the manifold 2 and the ground state 1.
• the pump power at which this ratio reaches its maximum value will in general depend on fibre design as well as on the size of the pump absorption cross-section from the ground-state and the upper laser manifold. From figure 2 it can be seen that, by launching 24mW of pump power into a single length of fibre of optimum length for the power launched a gain of 13dB would be achieved. However, if instead of launching all the pump power into one length of fibre, the pump power were split into four fractions, each of 6mW, and each of these four fractions were applied to a fibre of optimum length for 6mw power fibre, the total gain available would be 22dB. A significant improvement in pump efficiency of the exemplary amplifier has for the given available pump power thus been achieved.
• the gain can be maximised by splitting that pump power into a plurality of fractions, the pump power in each fraction being such that for the particular fibre, the ratio of optimum gain to launched pump power is a maximum, and launching each of the fractions into a separate length of fibre amplifier of optimum length.
• FIG 3 a fibre laser made in accordance with the invention is schematically illustrated.
• An optical fibre 41 has been notionally divided into four fibre amplifier sections 42, 43, 44,
• Section 42 extends from point a to point b, 43 from b to c, 44 from d to e and 45 from e to f.
• the fibre amplifier sections 42, 43, 44, 45 comprise lengths of erbium doped optical fibre, whereas the remaining sections of the fibre 41 comprise undoped lengths of optical fibre.
• the fibre 41 is formed by splicing lengths of doped fibre to lengths of undoped fibre in an appropriate manner to form a single length of fibre.
• Each of the three couplers is a dichroic fibre coupler.
• Each coupler may be formed by fusing together fibre 41 and a second fibre and by drawing each one by an appropriate amount in order that the coupler is capable of coupling pump radiation into the fibre 41 whilst leaving the input signal radiation 50 unaffected.
• Such couplers are well known and will not be described further.
• a pump fraction is longitudinally coupled into fibre section 42 via arm 51 in the same direction as the input signal radiation 50.
• a second pump fraction is longitudinally coupled via arm 52 into fibre amplifier section 43 such that it travels in the opposite sense to the input signal 50.
• a third pump fraction is longitudinally coupled via arm 53 into fibre amplifier section 44 in the same sense as the input signal.
• a fourth pump fraction is longitudinally coupled via arm 54 of coupler 49 into fibre amplifier section 45.
• An amplified signal 55 is thus produced.
• the fibre amplifier sections 42, 43, 44, 45 are arranged to be close to the optimum length for the particular power at which they are being pumped, by referring to a graph similar to that of figure 3 for the particular parameters of the system.
• the fibre sections must be short enough to ensure that the transparency point is not exceeded in order that there is always positive gain.
• the remnant pump from one fibre amplifier section combines with the pump fraction in the adjacent section to further maximise the gain.
• the fibre amplifier shown generally by the numeral 60 comprises three fibre sections 61, 62, 63 which are separate from one another.
• An optical pump source 64 such as an 807 nm Styrl 9M dye laser produces an optical pump signal which is longitudinally coupled into fibre amplifier section 62 by means of a dielectric mirror 65.
• An input signal is also coupled into fibre section 62 via fibre section 61 and mirror 65.
• a second optical pump signal is produced by the pump source 64 and longitudinally coupled into fibre section 63.
• the input signal, after having propagated through fibre section 62 is also coupled into fibre section 63.
• An amplified signal 67 is thus produced.
• optical is intended to refer to that part of the electromagnetic spectrum which is generally known as the visible region together with those parts of the infrared and ultraviolet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibre.
• the optical pump distribution is applicable generally to three or four level systems which exhibit ESA at certain pump wavelengths and is not restricted to erbium doped silica based fibre laser amplifiers of the type described above.

Landscapes

• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Engineering & Computer Science (AREA)
• Plasma & Fusion (AREA)
• Optics & Photonics (AREA)
• Lasers (AREA)
• Semiconductor Lasers (AREA)

Priority Applications (2)

Applications Claiming Priority (2)

Publications (1)

Family

ID=10652939

Family Applications (1)

Country Status (9)

Families Citing this family (7)

Citations (1)

• 1989
  • 1989-03-08 GB GB898905276A patent/GB8905276D0/en active Pending
• 1990
  • 1990-03-08 KR KR1019910701080A patent/KR920702050A/ko not_active Application Discontinuation
  • 1990-03-08 DE DE69031002T patent/DE69031002T2/de not_active Expired - Fee Related
  • 1990-03-08 EP EP90302510A patent/EP0387075B1/en not_active Expired - Lifetime
  • 1990-03-08 WO PCT/GB1990/000354 patent/WO1990010964A1/en active Application Filing
  • 1990-03-08 AT AT90302510T patent/ATE155289T1/de active
  • 1990-03-08 CA CA002047671A patent/CA2047671A1/en not_active Abandoned
  • 1990-03-08 JP JP2504258A patent/JP2927943B2/ja not_active Expired - Fee Related
  • 1990-03-08 AU AU51782/90A patent/AU650618B2/en not_active Expired - Fee Related

Patent Citations (1)

Also Published As

Similar Documents

Legal Events